The Global Positioning System (GPS) is a satellite based navigation system having a network of 24 satellites orbiting the earth 11,000 nautical miles in space, in six evenly distributed orbits. Each satellite orbits the earth every twelve hours. A prime function of the GPS satellites is to serve as a clock. Each satellite derives its signals from an onboard 10.23 Mega Hertz (MHz) Cesium atomic clock. Each satellite transmits a spread spectrum signal with its own individual pseudorandom noise (PN) code. By transmitting several signals over the same spectrum using distinctly different pseudorandom noise coding sequences the satellites may share the same bandwidth without interfering with each other. These coding sequences are a time series of bits, called “chips”. The code used in the non-military portion of the GPS system is 1023 chips long and is sent at a rate of 1.023 megachips per second yielding a time mark (i.e. a “chip”) approximately once every micro-second. The sequence repeats once every millisecond and is called the course acquisition (C/A) code.
Ground based GPS receivers may use a variant of radio direction finding (RDF) methodology, called triangulation, in order to determine the position of the ground based GPS receiver. The triangulation method depends on the GPS receiver obtaining a time signal from a satellite. By knowing the travel time of a GPS signal from a satellite to the GPS receiver, the distance from the receiver to the satellite can be calculated. If, for example, the GPS satellite is 12,000 miles from the GPS receiver then the receiver must be somewhere on the location sphere defined by the radius of 12,000 miles from that satellite. If the GPS receiver then ascertains the position of a second satellite it can calculate the receiver's location based on a location sphere around the second satellite. The two spheres intersect and form a circle, and so the GPS receiver must be located somewhere within that location circle. By ascertaining the distance to a third satellite the GPS receiver can project a location sphere around the third satellite. The third satellite's location sphere will then intersect the location circle produced by the intersection of the location spheres of the first two satellites at just two points. By determining the location sphere of one more satellite, whose location sphere will intersect one of the two possible location points, the precise position of the GPS receiver is determined. As a consequence, the exact time may also be determined, because there is only one time offset that can account for the positions of all the satellites.
There are multiple ways of using the radio spectrum to communicate. For example, in frequency division multiple access (FDMA) systems, the frequency band is divided into a series of frequency slots and different transmitters are allotted different frequency slots. In time division multiple access (TDMA) systems, the time that each transmitter may broadcast is limited to a time slot, such that transmitters transmit their message one after another, only transmitting during their allotted period. With TDMA, the frequency upon which each transmitter transmits may be a constant frequency or may be continuously changing (frequency hopping). A third way of allotting the radio spectrum to multiple users is through the use of code division multiple access (CDMA) also known as spread spectrum. In CDMA all the users transmit on the same frequency band all of the time. Each user has a dedicated code that is used to separate that user's transmission from all others. This code is commonly referred to as a spreading code, because it spreads the information across the band. It is also referred to as a pseudorandom noise (PN) code.
To decode the transmission at the receiver it is necessary to “despread” the code. The despreading process takes the incoming signal and multiplies it by the spreading code and sums the result. This process is commonly known as correlation, and it is commonly said that the signal is correlated with the PN code. The result of the despreading process is that the original data may be separated from all the other transmissions, and the original signal may be recovered.
Jamming interference from a variety of sources can disrupt the process of finding and decoding GPS signals, which can cause errors. These errors may build up if jamming continues. As a result of these jamming-induced built up errors, significant errors in position may occur at the GPS receiver.
In particular for spread spectrum systems such as GPS, it is possible for a narrowband signal to jam a GPS receiver. Due to the nature of the demodulating process in a GPS receiver, a strong signal going into a despreader results in a strong signal coming out of the despreader. A receiver uses the despreader output downstream for four primary purposes: 1) to demodulate transmitted data, 2) to determine the signal strength of the spread spectrum signal, 3) to determine if the receiver has acquired the GPS signal, and 4) to maintain a lock on the signal in order to continue to track the signal. If a narrowband signal enters the despreader, the high amplitude output of the despreader can deceive the receiver into thinking it is still properly tracking the spread spectrum signal.
Prior art methods for detecting jamming includes frequency based analysis of a spread spectrum signal at the pre-detection stage, for example analyzing the frequency-domain representation of the received signal. This requires extensive use of the Fast Fourier Transform (FFT), which burdens the processor and memory of a receiver as the FFT output over multiple time intervals must be stored during the processing of information. A new method of jamming detection, which can be readily added to an existing receiver with no additional hardware requirement and impose a minimum burden on the processor and memory is desirable.
Accordingly, there is a significant need for an apparatus and method that overcomes the deficiencies of the prior art outlined above.
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